Money Trail May Mark Path of New Pandemics

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Advise patients who ask that this study implies that a pandemic will spread in leaps and bounds, rather than slowly crossing the nation at a well-defined rate.

Note that, while researchers fear a pandemic influenza strain might erupt soon, this is based on historical patterns of disease and not on clinical evidence.

GOTTINGEN, Germany, Jan. 25 - The battle cry of epidemiologists planning for the next flu pandemic might be the immortal words of actor Cuba Gooding Jr. in the movie Jerry Maguire -- "Show Me The Money!"

Scientists here and in California have developed a mathematical model of how humans travel in the jet age -- something that was difficult to do but was considered a prerequisite in understanding how a new flu pandemic might spread.

And it all came down to cash, they reported in the Jan. 26 issue of Nature.

The model is based on an analysis of data collected by a popular Internet game -- found on the Web site www.wheresgeorge.com -- in which participants enter the serial numbers of bills in their possession.

Over time, as different people enter the same bill, the game builds up a picture of how the money is moving, said Dirk Brockmann, Ph.D., of the Max Planck Institute for Dynamics and Self-Organization here.

But since paper money -- like viruses -- travels with people, the game also allowed the researchers to model how humans move through the world, without actually tracking them, Dr. Brockmann said

"The enormous amount of data, as well as the geographical and temporal resolution of bill-tracking, allows us to draw conclusions about the statistical characteristics of human travel," he said.

Historically, pandemics have moved relatively slowly, because human travel was limited. The 14th century pandemic of bubonic plague - the Black Death -- took three years to hit the entirety of Europe, moving from south to north with an average rate of spread of slightly more than a mile a day.

"But today people move great distances in short time periods, as well as short distances, and they use variable means of transportation," said co-author Lars Hufnagel, Ph.D., of the Kavli Institute for Theoretical Physics at the University of California in Santa Barbara.

"We can expect that future pandemics will spread according to other rules, and more quickly," he said.

But uncovering those rules seemed difficult if not impossible -- today people travel short distances using bicycles, subways, cars, streetcars, and even rollerblades, while long distances are covered by buses, trains and airplanes.

To get a good picture would need long-term monitoring all those means of travel, the researchers noted.

"Since we can't track people with tracking devices, like we do animals, we needed to get data that provided us with millions of movements of individuals," Dr. Hufnagel said.

The money trail helped provide the answers, he said.

Until now, he said, most models of the spread of disease were based on the assumption that pathogens spread in waves, with a sharply defined upper limit on how fast they could travel.

These models do quite well in describing the wavelike diffusion of historical pandemics, he said. But the new results suggest that pandemics will be "superdiffusive" -- they'll be able to jump across huge swaths of territory without crossing the intervening space.

For example, he said, a pandemic that started in New York would follow the old rules for a while, spreading in waves in a confined area. But sooner or later, an infected person would hop on a plane and go to Los Angeles -- starting another local pandemic.

Meanwhile, towns and cities between the two might not be affected at all.

In fact, that's what happened in the 2003 SARS outbreak, which started in China, jumped to Hong Kong, and then leapt several thousand miles to Toronto in Canada, Dr. Hufnagel said, while intervening sites were barely affected.

The research should "serve as a starting point for the development of a new class of models for the spread of human infectious diseases, because universal features of human travel can now be accounted for in a quantitative way," the researchers concluded.

Reviewed by Robert Jasmer, MD Assistant Professor of Medicine, University of California, San Francisco